At the large scale, and for a range of emission scenarios produced for the
IPCC's Third Assessment Report (see Section 3.8),
climate model results suggest that North America could warm at a rate of 1-3°C
over the next century for a low-emissions case (B1). Warming could be as rapid
as 3.5-7.5°C for the higher emission A2 case. Even the B1 case suggests
substantially more warming over the next century than we have observed over
the past 100 years. In addition, we know that the past century saw changes in
temperature, precipitation, and other variables. Climate model predictions of
precipitation remain highly uncertain. Many models suggest higher rainfall over
North America accompanying warming in simulations of the IS92a emission scenario
(Schimel et al., 1996). While some models suggest widespread and substantial
increases in rainfall over most of North America, other models suggest a weaker
increase in rainfall.

Understanding concurrent changes in regional temperature and precipitation
is crucial. Warming with increasing precipitation is likely to increase plant
growth, which may increase carbon storage (VEMAP Members, 1995) and may increase
pest and pathogen invasion and expansion. Warming with a lesser increase or
a decrease in precipitation could cause direct vegetation mortality and increase
the risk of wildfire. Variability also plays a role. General increases in temperature
and precipitation might increase plant growth, but occasional severe droughts
would then maximize the chances of wildfire. Preliminary results from modeling
of the United States suggest that projected changes in climate will cause very
substantial changes to the distribution and productivity of ecosystems and to
disturbance regimes (fire and drought probabilities). Subtropical conditions
will extend further north into the United States, with accompanying changes
to vegetation, hydrology, and the potential for disease. Changes at the Arctic
border suggest changes to the forest-tundra transition region, losses of permafrost,
and an altered growing season.

Concern about the spatial uncertainty of model-based climate scenarios has
led to various attempts at downscaling global scenarios to regional scales.
This can influence the results of impact studies, as illustrated by a recent
case study of crop yields in the U.S. Great Plains (Mearns et al., 1999).
Furthermore, published regional impact studies have used older global climate
scenarios similar to the temperatures resulting from the new emissions cases,
but these may not be directly comparable.